Advances in science are often achieved with the development of technology that allows scientists to differentiate and refine concepts which were previously undifferentiated. The field of obesity research has progressed from simple measurement of weight loss to multi-compartment models of fat, fat-free mass and regional distribution of fat. An opportunity for further refinement now presents itself in that magnetic resonance imaging technology can be used to image individual organs and tissues. This presents the opportunity to investigate important unresolved questions. For example, information is lacking on the effects of weight loss on individual organs and tissues. The loss of fat free mass (FFM) cannot fully explain the reported decrease in resting energy expenditure (REE) that accompanies weight loss. Since the FFM compartment consists of numerous tissues and organs, each with a different oxidative metabolic capacity, the possibility exists that individual vital organs may change disproportionately to the change in total FFM with weight loss, thereby explaining reductions in REE post weight loss. Complete organ-tissue volume and mass reconstruction of most major compartments is now possible, using magnetic resonance imaging (MRI). The proposed ancillary study will investigate the composition and nature of weight loss and weight maintenance during the Look AHEAD trial. Secondary research questions relate to modeling changes in resting energy expenditure and changes in cardiac structure and function associated with weight loss. The Specific Aims are to: 1) describe for the first time the composition of weight loss in diabetic subjects at the tissue/organ level of body composition, specifically, we wish to determine whether and to what degree different tissues and organs are conserved or lost in comparison to total FFM change during weight loss; 2) assess the effects of aging on organ and tissue mass in 50+ year old diabetic men and women in a control group, and 3) test whether a portion of weight loss related changes in REE can be explained by changes in the relative fractions of body mass as high (brain, heart, liver, kidney) and low (skeletal muscle, adipose tissue) metabolic activity tissues with respect to oxidative metabolism in 50 plus year old diabetic men and women. A secondary aim is to assess the effects of weight loss on cardiac structure and function using cardiac gated MRI. Consenting, eligible Look AHEAD participants at St. Luke's and at the University of Pittsburgh will participate in MRI and REE tests. Seventy men and women will be measured, equally distributed between the two sites, and equally distributed between Lifestyle Intervention group and a Community Care group. Measures will be acquired at baseline, following one year of intervention (Year 1), and following one year of maintenance (Year 2), to produce a 2 X 3 factorial design. The proposed studies promise to answer important body composition related questions regarding the composition of weight loss at the organ/tissue level.